The invention relates to a method and a device for adjusting the PID characteristics of controller compensating networks, particularly in hydraulic pulsing machines and a method for monitoring changes in time on test specimens examined by means of hydraulic pulsing machines.
Hydraulic pulsing machines are usually used for determining and investigating the elasto-mechanical behavior of test specimens. They are known to contain a controller compensating network which requires careful adjustment before each examination. The aim is considered to be that of achieving good agreement in the time characteristic between the set-value quantity and the controlled quantity. For this, the PID characteristics of the controller compensating network must be suitably adjusted. Since the transfer characteristics of the controlled system are a function of the test set-up/test specimen, it can be recognized that the PID setting of the compensating network must be adjusted to the respective test set-up/test specimen for each test.
The method most frequently used in practice consists in adjusting the compensating network with the aid of step functions. In this method, the control loop is supplied with a step function via the set-value input and the variation in time of the response of the controlled quantity to the step signal is observed. The PID setting of the compensating network is considered to be satisfactory when the controlled quantity settles in minimum time at the final value of the step and the overshoot displayed by the controlled quantity around the final value of the step is small.
These two demands are contradictory. The first condition requires a high control-loop gain which, however, reduces the stability in the control loop and thus leads to unfavorable overshoot characteristics, thus contradicting the second requirement. In consequence, the demands with respect to good control accuracy and good stability of the control loop differ. It is frequently found to be difficult to achieve optimization during the adjustment of the PID characteristics of the controller compensating network with the aid of step functions. This applies especially if the test set-up/test specimen involves more complicated elasto-mechanical structures. Mention is made, for example, of the case where a test specimen displays, in a frequency range of interest, one or several characteristic frequencies which are attributable to its natural structural characteristics of vibration. In such difficult cases, even the experienced test engineer frequently lacks an understanding to draw conclusions concerning the optimum setting of the compensating network only by means of the step response of the controlled quantity. The reason for this is that the information content of the step response signal is too low, especially in these difficult cases.
A further disadvantage in adjusting the compensation network via step functions has also been found to be that the hard step signal can destroy the test setup/test specimen. The recommendation of the manufacturers of the hydraulic pulsing machines to carry out the PID adjustment in this case with dummy test specimens can often not be implemented for economic reasons because of the structural complexity of the test set-up/test specimen. In addition, the gradient of the force signal generated by the hydraulic pulsing cylinder cannot be considered to be infinitely steep. Because of the fact that the step function is not ideally steep, it is thus only possible to excite the test set-up/test specimen within a limited lower frequency range. It is quite conceivable that a high-frequency instability of the system will remain undiscovered during a step excitation.